1. Field of the Invention
The present invention relates to a method for preparing a preform for a composite material, and specifically, to a whisker-preform for use as a fibrous skeleton in manufacturing whisker-reinforced composite materials which are to be infiltrated or impregnated with matrix materials such as metals, alloys, plastics and the like.
2. Description of the Related Arts
Various whiskers which are composed of a single crystal such as silicon carbide (SiC), alumina (Al.sub.2 O.sub.3), graphite (C) or silicon nitride (Si.sub.3 N.sub.4) have a thickness of below 100 .mu.m and a length to diameter ratio of at least 100. The whiskers are mineral fibers of a single crystal, and have high strength, high modulus of elasticity and high heat resistance, and have stable chemical properties. Therefore, usage thereof as a reinforced composite for metals or plastics has been attractive.
For a method for preparing a fiber-reinforced composite using a whisker such as SiC, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, graphite as a reinforced material, a method in which a whisker-preform is first prepared, a matrix material of a molten metal or plastic dissolution is impregnated with a whisker-preform under pressure and then solidified is commonly used. The quality of a composite material prepared by said method largely depends upon texture structure of the whisker-preform to be used.
Recently, as various composite products have been produced on a large scale, and therefore, it has become very important to prepare quickly and finely the whisker-preforms to be used as intermediates.
As conventional method for preparing a whisker-preform, a method for preparing a whisker-preform in which a whisker is dispersed into water or an organic solvent such as alcohol, acetone, ether, and the resulting dispersion is filtered, and the product is formed and dried, is shown in Japanese Patent No. Hei 4-30448. Japanese Patent No. Hei 4-30448 describes that SiC whisker is added to the organic solvent to obtain a concentration of 10 to 60 g/l by mixing with stirring, and uniformly dispersed into the organic solvent, and filtered to remove partially the organic solvent, and subsequently metal powder is added to filtrate, and adjusted to a final concentration of 350 to 1100 g/l, thereafter, the mixture is filtered under suction and molded to obtain the filter cake, and dried. However, the method has following problems.
When the organic solvent is used as a dispersant, carbon particles are produced on the surface of the whisker. Accordingly, when it is complexed with a metal, it gives carbide having very weak bonds thereby decreasing mechanical properties of the composite product.
Moreover, the above method is for preparing a preform composed of a whisker and a single fiber, and it has been difficult to produce a preform having high volume fration because metal particles added are close-packed.